63amp RCD protection linked to 100A consumer unit

robbie77 · 8 Mar 2010

Hello,

I was looking at the consumer unit at my parents house. They recently had a kitchen refurb and some electrical work done including a new consumer unit I think its an MK 17 edition type.

It has a TT earth setup and the tails go from this 63Amps rcd into the consumer unit, the main switch says 100A

The electrics work without issue but I was wondering if the RCD should be upgraded maybe at leat to 80Amp which is what the the RCD's in the board are.

Does it still need the extra RCD as the board has them built in or did I read it was a requirement for TT earth setups?
Cheers

bongos · 8 Mar 2010

robbie77 said:
The electrics work without issue but I was wondering if the RCD should be upgraded maybe at leat to 80Amp which is what the the RCD's in the board are.

I assume from your description that there are two RCD's in the new CU? If so and the circuits are split evenly across both then you are unlikely to ever see more than 63A drawn through either one.

robbie77 said:
Does it still need the extra RCD as the board has them built in or did I read it was a requirement for TT earth setups?

It sounds like you are saying that the original RCD was left in when they installed the main board? This may have been left as it would have been the only way to isolate the system for the CU change without removing the main fuse.

Assuming your new CU is not metal clad then if the old "upfront" RCD is of the 30mA variety then it should be removed as there will be discrimination between this RCD and the two in the new CU.

If the old RCD is a time delayed "Selective" type 100mA RCD then it is ok to leave it in place.

mfarrow · 8 Mar 2010

robbie77 said:
the main switch says 100A

What does your cut-out say (the box before the meter)? If nothing no hard ringing your electricity supplier to check.

robbie77 · 8 Mar 2010

I will have to check on the cut out next time I am there mfarrow. Cheers

Bongo there are I think 2 RCD's on the split rail yes.
I think it migh be the upfront RCD is of the 30mA variety, if it is then maybe the electrician should come back and remove it as it sounds like it does not need it, does that mean the tails will then go straight into the CU?

He may need to contact the Electricity company then as I think its is the isolating switch!

Thanks for the help

bongos · 8 Mar 2010

robbie77 said:
I think it migh be the upfront RCD is of the 30mA variety, if it is then maybe the electrician should come back and remove it as it sounds like it does not need it, does that mean the tails will then go straight into the CU?

Yep. Main tails from meter staright into CU. As I said my guess as to why it was left in place might have been that it saved him snipping the seals on the main fuse. Did he replace/upgrade the tails between the meter and the old RCD if they were undersized?

If the old RCD is 100mA 'S-type' though I think I would probably leave it in place.

Taylortwocities · 8 Mar 2010

If its 30mA it will probably not be an S type.

Post us a picture but in my view that RCD should go as it now constitutes a single point of failure (bad, bad design). But it does depend on the config of the consumer's unit.

Click-Sure · 9 Mar 2010

Does it look like this?

Untitled

Click-Sure
22 Jan 2010
2

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63amp RCD protection linked to 100A consumer unit

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